A packaging bag typically includes a seam, a manufacturer end, and a customer end. Bags are manufactured in a variety of ways, which may include sewn open mouth, pasted open mouth, and pinch bottom open mouth. To manufacture a bag e.g., a heavy Kraft paper bag by a method known as pinch bottom open mouth, a sheet or multiple sheets of Kraft paper or film substrate is first fabricated as a tube with an adhesive-sealed seam. Then, an adhesive is applied to the manufacturer end and sealed by the bag manufacturer. The adhesive is also applied to the customer end, which will be heat-sealed by the customer once the products are packaged into the bag. This type of bag is also called a pinch bottom open mouth bag.
Multiwall paper bags have been used for heavy-duty packaging applications such as packaging grain, fertilizer, chemicals, pet food, etc. Although formed from a plurality of paper plies, these paper bags have relatively high tear/wear rate and can be easily torn open and damaged during transportation, especially when the bags are filled with heavy products.
The packaging industry has been evaluating woven polypropylene (WPP) substrates for the manufacture of a variety of end use bags, especially heavy-duty bags. Woven polypropylene bags offer the best packaging for a large variety of products in industries such as agriculture, chemical, mining, building materials, pet food, and construction. Whether simple bags for business-to-business applications or bags with high-quality graphics for superb consumer appreciation, woven polypropylene bags offer economy, better protection, weight savings, and greater strength than other available packaging. Especially over paper bags or jute bags woven polypropylene bags offer lighter weight, greater bursting strength, better pest resistance, resistance to tearing, reusable, properties that do not degrade if wet, and are less expensive.
Currently, woven polypropylene bags are manufactured with a hemmed top (at customer end) and a fold stitched bottom (at manufacturer end), along with single or double sewn seams. The sewn woven polypropylene bags are hard to make and also costly due to the lower production rates and the cost of inline sewing equipment. Efforts have been made to develop a suitable adhesive composition for bonding WPP substrate; but none of the conventional hot melt adhesives, such as, polyamide, ethylene vinyl acetate, or polyethylene based adhesives could provide adequate bonding because the WPP substrate is non-porous and has low surface energy, e.g., a surface energy value of no greater than about 30 dyne. In addition to the above surface bonding difficulty, none of the conventional adhesives could bond adequately to the WPP substrate at a lower temperature e.g., about 0° F. and maintain the bonding ability at a higher temperature, e.g., about 140° F. Further, the WPP substrate tends to have a heat distortion at a temperature higher than about 330° F., which also requires an adhesive suitable for the low temperature application.
Therefore, there is a substantial need for an adhesive composition especially capable of excellent adhesion to a woven polypropylene substrate.